Most structural steels are quenched and tempered steels that are realized by increasing toughness and strength through reheating, quenching, and tempering after hot working. On the contrary, a non-heat treated steel is a steel that does not undergo heat treatment after hot working, i.e., a steel having a similar toughness and strength to quality of the material to be heated (heat treated) can be obtained. The name of steel that can be used without heat treatment is a non-heat treated steel also known as micro-alloyed steel, in which the quality of the material is achieved by adding a very small alloy. Hereinafter, the steel having the properties as mentioned above will be known as a non-heat treated steel in the present invention.
Generally, a wire rod is produced as a final product by using the following steps. The final product of the wire rod can be produced in the order of Rolling Rod→Cold Drawing→Spheroidization Heat treatment→Cold Drawing→Cold Forging→Quenching and Tempering→Product. However, the non-heat treated steel is produced in order of Hot Rolling Rod→Cold Drawing→Cold Forging→Product. Therefore, the non-heat treated steel can be produced as an economical product without heat treatment process. At the same time, a final quenching and tempering steps are not performed. Therefore, the non-heat treated steel has been applied in many products due to the securing of linearity caused by not generating a heating deflection, i.e., a defect caused during the heating.
However, when the steps are processed, the strength of the product is further increased, while the toughness is continuously decreased, because the heat treatment process is omitted and cold working is continuously applied. Therefore, domestic and foreign manufacturers of a wire rod have been focused on the technology for manufacturing a non-heat treated steel with excellent toughness that has improved the toughness of non-heat treated steel. The methods for manufacturing the non-heat treated steel are methods for refining a steel grain by using a precipitate, a method for securing a composite microstructure by adding alloy elements, and the like.
Japanese Patent Laid-Open Publication No. 1995-054040 discloses a method for providing a non-heat treatment steel wire rod with 750-950 MPa of tension by hot rolling the alloy steel that is composed of C: 0.1˜0.2%, Si: 0.05˜0.5%, Mn: 1.0˜2.0%, Cr: 0.05˜0.3%, Mo: 0.1% or less, V: 0.05˜0.2%, Nb: 0.005˜0.03%, and the remainder Fe, as a percentage by weight, cooling the alloy steel within 60 sec between 800˜600° C. for a cooling step, and heating at 450˜600° C., or cooling the alloy steel after continuously maintaining it for at least 20 minutes at a temperature of between 600˜450° C., and then cold working. However, the product is hot-rolled through a process, known as controlled rolling, and expensive components such as Cr, Mo, V, and the like are added in the method as mentioned above, so that it is uneconomical in use.
In addition, Japanese Patent Laid-Open Publication No. 1998-008209 relates to non-heat treated steel with excellent strength after hot working, and excellent cold formability and a method for manufacturing the same, and a method for preparing a forging member by using a non-heat treated steel, and also relates to non-heat treated steel with excellent cold formability, in which a volume of a ferrite phase is at least 40%, and a hardness is 90 HRB or less, for the steel having a controlled contents of C, Si, Mn, Cr, V, P, O, S, Te, Pb, Bi, and Ca. Specifically, the document relates to a method for continuously cooling to a temperature of Al point or less at cooling rate of 120° C. or less per minute immediately after hot-rolling to be 800˜950° C. during a final working temperature, a method for cooling a hot rolled steel material in the air after heating for at least 10 minutes at 800˜950° C., and also a method for preparing a structural member with 20˜35 HRB of hardness by cold working or warm working at a temperature of 600° C. or less, preparing a preform, and cooling at the air after hot-forging the preform at 1000° C.˜1250° C. However, the technology is limited to specific steel containing elements that are usually not used, and is not applied to cold forging.
In addition, Japanese Patent Laid-Open Publication No. 2006-118014 provides a method for manufacturing case-hardened steel that is suitable for a bolt, and the like, which suppresses grain coarsening after heat treatment, even if cold formability is excellent and also the working with a high cut rate of expanded line is performed. The method as mentioned above uses the steel material that is composed of C: 0.10˜0.25%, Si: 0.5% or less (except 0%), Mn: 0.3˜1.0%, P: 0.03% or less (except 0%), S: 0.03% or less (except 0%), Cr: 0.3˜1.5%, Al: 0.02˜0.1%, N: 0.005˜0.02%, the remainder Fe, and other inevitable impurities, as a percentage by weight, and the method for manufacturing non-heat treated wire rod with excellent toughness is achieved by performing hot finish rolling or hot finish forging at 700˜850° C., then cooling by up to 600° C. at a cooling rate of 0.5° C./sec or less, and suppressing below 20% of cut rate of expanded line by cooling to room temperature. The technology as mentioned above is uneconomical due to the use of expensive Cr.